Electrically heated furnace utilizing ceramic heating elements

ABSTRACT

An electrically heated furnace utilizing ceramic heating elements. The heating elements are series-connected across a spark gap and take on a variety of different configurations depending on their specific application.

United States Patent [1 1 Kuhlmann-Schafer et al.

[ ELECTRICALLY HEATED FURNACE UTILIZING CERAMIC HEATING ELEMENTS Inventors: Wilhelm H. Kuhlmann-Schafer, Hannover; Klaus Hagen, Peine; Hans-Dieter Peters, Schmedenstedt; Gunther Wolters, Edemissen, all of Germany Assignee: Preussag AG., Hannover, Germany Filed: July 9, 1973 Appl. No.: 377,387

[30] Foreign Application Priority Data July 25, 1972 Germany 2236286 US. Cl. 13/25 Int. Cl. H05b 3/14 Field of Search l3/25;3l3/34l; 338/223,

[451 Mar. 26, 1974 [56] References Cited UNITED STATES PATENTS 3,345,597 10/1967 Schrewelius et a1 13/25 X 3,371,142 2/1968 Soller 13/25 3,397,375 8/1968 Casper et a1 13/25 X 3,571,476, 3/l97l Anthony et a1. 13/25 UX Primary ExaminerRoy N. Envall, Jr,

Attorney, Agent, or Firm-Molinare, Allegretti, Newitt & Witcoff [57] ABSTRACT An electrically heated furnace utilizing ceramic heating elements. The heating elements are seriesconnected across a spark gap and take on a variety of different configurations depending on their specific application.

12 Claims, 7 Drawing Figures PATUHEB A I974 .SHEEI 1 [IF 2 Fig. 2

ELECTRICALLY HEATED FURNACE UTILIZING CERAMIC HEATING ELEMENTS BACKGROUND OF THE INVENTION In an article entitled Design and Performance of Electric Furnaces with Oxide Resistors in the magazine Journal of the American Ceramic Society Vol. 33, 1950, pp. 333 to 339, an electrically heated furnace having a cylindrical ceramic, vertically arranged heating element is discussed. At its axial ends, this cylindrical heating element is connected with a cource of current. Such resistance-heated furances have the considerable disadvantage of a negative temperature coefficient (i.e., as the temperature increases, the resistance decreases). Thus, instability results when the furnace is operated with a constant voltage source. This instability may result in undesired cooling-off or in undesired overheating and destroying of the furnace. In the article it is specified that there is no compromise. This disadvantage can only be overcome either by incorporating a large compensating resistance which causes significant performance loss or by the use of an expensive control device to regulate the input voltage and/or current as a function of the temperature. Because of the negative temperature coefficient of the ceramic heating elements, it is also not possible to connect several heating elements in parallel without making separate provisions for the stabilization of each element. Therefore, a practical application of the parallel-connection is out of the question.

A further disadvantage of a furnace with a cylindrical ceramic heatingelement is that canalization of the current (i.e., distribution of current unequally over the cross-section of the conductor) takes place. This effect is explained in the same manner asthe effect occurring in the case of the parallel-connection of heating elements. This canalization does not only result in a nonuniform heat production, but as a consequence thereof also in overheating and destruction of the heating element. For this reason, such furnaces have up to now only been manufactured in very small sizes.

In the German Patent Application (01.8) 1 966 175 (Oct. 14, 1971), an electrically heated furnace is.

shown which uses a ceramic heating element. This element is also cylindrical, but is partially slit in axial direction. The current flows in one half of the cylinder to a terminal point and returns therefrom into the other half. Through this design, the canalization danger is reduced, but by no means eliminated. Also, this small advantage is more than outweighed by the extremely complicated shape of the heating element and the danger of breakage caused thereby. An increased danger 1 through canalization still exists in the area where the current flows around the ends of the slots. In this area,

current density is increased, and there is also a canalization which in turn results in the dangers and disadvantages previously explained.

In the article first discussed, an electrically heated furnace is shown which uses ring-shaped ceramic heating elements which are inductively fed by an induction coil located on the outside. The article explains that this kind of inductive feeding does not produce any instability and attributes this to the fact that the power consumption of the hot furnace decreases with increasing temperature because simultaneously the penetration of the magnetic field decreases with decreasing resistance at increasing temperature. However, the inductive feeding is extremely expensive since it requires a high-frequency generator. Moreover, it requires special measures for blocking the sweep radiation. Thus the increased stability achieved is more than counterbalanced by the increase in the complication of the feeding source.

SUMMARY OF THE INVENTION It is the purpose of the present invention to construct a furnace electrically heated by means of a heating element of ceramic material which may be supplied with power from a constant voltage source without producing significant instabilities.

This invention solves the problems previously mentioned by the use of at least two heating elements connected in series with one another over a spark gap.

Through the interposition of spark gaps which are, for example, very simply formed between heating elements placed on top of one another, a stable operation can be attained.

A further development of the invention relates to placing parts of the heating elements on top of one another and providing roughened surfaces on those parts which contact one another. Through these roughnesses, the spark gap can be enlarged. Naturally, it is also possible to provide an appropriate size of the spark gap through appropriate spacing.

A further development of the invention relates to utilizing heating elements shaped as elongated bars, preferably rectangular in their cross-section. In the case of this embodiment, one avoids, at least partially,'a canalization. The elongated shape of the heating elements allows the current to flow forcibly through the entire cross-section. In addition, this shape avoids the formation of thermal stresses to a great extent since the bars can freely move with their ends.

A further development of the invention relates to the use of bar-shaped heating elements which are bent in annular fashion, but which do not touch with their ends. Consequently, these bars bent in ring-shaped fashion enclose the interior of the furnace and consequently also determine its shape. Naturally, an arbitrary stacking of elongated heating'elements with arbitrary longitudinal shape for the producing of any shapes whatsoever of the interior of the furnace is possible at any time, too.

A further development relates to the use of the barshaped heating elements which are stacked on top of one another and which are alternately connected with one another in meander-shaped fashion by their ends through conductive connecting-parts, which connecting-parts are preferably formed of the same ceramic material as the heating elements. This shape can be produced in particularly simple manner. The connecting-parts consist for example simply of ring-shaped heating elements sawed apart.

A further development of the invention relates to the use of connecting-parts which are respectively made out of one piece with one of the ring-shaped heating elements to be connected which structure is thus, when unwound, essentially L-shaped. in the case of this embodiment, only one spark gap is respectively present between adjacent heating elements. Consequently, an

adaptation of the resistance behavior is possible. Simultaneously therewith, the position of the connectingparts is fixed.

A further development of the invention relates to the employment of bar-shaped heating elements which are kept separated through insulating range spacers. Thereby these heating elements are supported in such a manner that even with very high temperatures when their stability is reduced, they are sufficiently supported. This is of special importance in cases where many heating elements are stacked on top of one another.

A further development of the invention relates to a ring-shaped construction of the heating elements where in addition to a connecting-part also serving for spacing, two insulating range spacers are provided preferably at equal peripheral distances. In this manner, the range spacers and the connecting-part form a threepoint contact which secures a particularly stable support of the heating elements. Expediently, the range spacers and the connecting-parts which have the same function are located in one row on top of one another so that the heating elements are not stressed through bending through the weight of the superimposed heating elements.

A further development of the invention relates to the use of bar-shaped heating elements which form halfrings stacked on top of one another in the manner of a helix, partially overlapping one another, in which case between the individual windings of the coils, insulating range spacers are set up. As a rule a greater distance between heating elements set up on top of one another results in this case. The current does not proceed in meander-shaped manner, but in the manner of a spiral. In this embodiment, a further development relates to the heating elements which form half-rings in which two adjacent half-rings of a coil-shaped arrangement rest respectively upon one another with their front surfaces.

A further development of the invention relates to the use of insulating range spacers which are U-shaped, the legs being set up between the heating elements to be kept at a distance while the cross-parts of the U-shape extends in the direction away from the furnace. The result of this'shape and position of the range spacers is that the cross-parts of the U-shaped range spacers extend into a cooler range in which they have a temperature at which a high insulation resistance can be attained with ceramic material.-

BRIEF DESCRIPTION OF THE DRAWINGS There follows a brief description of the drawings showing several presently preferred embodiments of the present invention wherein like numerals refer to like elements and wherein:

FIG. 1 is a perspective view of a first preferred embodiment of the invention with ring-shaped heating elements stacked in meandershaped manner;

FIG. 2 is a perspective view of a second preferred embodiment of the invention with heating elements shaped like half-rings and stacked in the manner of a coil;

FIG. 3A is a schematic diagram of the heating elements set up in a horizontal spiral, and FIG. 3B is a perspective view of a half-ring heating element having plates on its front surfaces;

FIG. 4 is a cross-sectional view of another preferred embodiment of this invention with L-shaped heating elements set up in one plane;

FIG. 5 is a perspective view of one of the L-shaped heating elements of FIG. 4;

FIG. 6 is a top elevational view of an insulating range spacer according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 11, several ring-shaped heating elements 1, 2 and 3 are stacked on top of one another with a space between one another. Their inside space forms the furnace space. Between ends 4 and 5 of heating elements 1 and 2, a connecting-part 6 has been placed which, through sawing-apart of a heating element, corresponds in cross-sectional shape to one of the heating elements 1 to 3. In the same manner, between ends 7 and 8 of heating elements 2 and 3, a connecting-part 9 is placed. Consequently, the current flows through the stacked heating elements in meander-shaped fashion in the direction of arrows 10, ill, 12, and 13.

The spark gap is created. between the adjacent sur-' faces of elements 1 and 6, elements 2 and 6, elements 2 and 9, and elements 9 and 3. This spark gap is an actual space between the elements. Because the surface of the heating elements is not smooth, there are several points of contact between the overlapping adjacent surfaces. However, the resistance of these contacts is very high because the cross-sectional area of contact is very small. Thus the effective resistance is established only by the spark gap or non contacting portions of the overlapping adjacent surfaces.

' which is stabilized is suitable.

FIG. 2 shows heating elements 20, 21, 22, 23 and 24 shaped in the manner of half-rings which are placed on top of one another with their ends in such a manner that they overlap one another and together form a coilshaped arrangement through which the current can flow in the direction of arrows 25 to 28. Consequently, in this embodiment connecting-parts are omitted. In the range of overlappings, range spacers 29 and 30 are set up which space apart the windings of the coil shaped arrangement.

FIGS. 3A and 3B shows on the left side a heating element 31 with the shape of a half-ring which heating element shows additionally, but not necessarily plates 32 and 33 on its front surfaces. On top of these plates which form connecting-parts, several heating elements are set up in coil-shaped manner corresponding to the illustration of FIG. 3A. Thus, heating elements rest with their front surfaces on the front surfaces of heating elements located underneath.

FIG. 4 shows another embodiment in which L-shaped heating elements 34) are stacked on top of one another in such a manner that the current flows in meandershaped fashion in the direction of arrows 35. The short legs of the L-shaped heating elements serve as the connecting-parts.

Between adjacent heating elements where there are no short legs which serve for support, range spacers 36 are provided. At the very bottom of the arrangement two range spacers 36 are set up in order to provide height compensation corresponding to the leg length or width. Naturally, the length of the short legs is arbitrary so that the power density is varied. The spark gaps are respectively formed between the support surfaces between the short legs and the long legs of adjacent heating elements.

FIG. 5 shows an L-shaped heating element 34 according to the arrangement in FIG. 4. In this illustration, a square front surface 37 is visible which, together with the opposite surface of the long leg of an adjacent heating element 34, forms the spark gap.

FIG. 6 shows a range spacer 38 which is designed in U-shaped fashion and which shows a cross-part 39 as well as legs 41 and 42 which space two heating elements 43 and 44 which are only shown with their front surfaces. The U-shaped range spacer 38 extends with its cross-part 39 away from the interior of the furnace,

. consequently into a cooler zone. Thus the range spacer 38 consisting of ceramic material has in the range of its cross-part 39 in correspondence with the known resistance behavior of ceramic material a high resistance and therewith good insulating qualities.

EXAMPLE A furnace was constructed according to FIG. 1. In

this case, the heating elements and the connecting parts consisted of ZrO The heating elements had an inner diameter of 50 mm, a height of 22 mm and a wall thickness of 8 mm. The height of the connecting-parts amounted to 20 mm.

After pre-heating of the furnace through a separate heating element to approximately 1,300C, a voltage was applied which permitted the flowing of enough current in order to increase the temperature of the individual heating elements. With increasing temperature, first of all the current flow was increased. In order to avoid too fast of an increase of the temperature, the voltage was reduced, and that is to say at an interval of minutes. In this case, the results were as follows:

Temperature(C) Voltage (V) Current Intensity (A) 1280 400 4 1310 415 4,3 1400 380 5 1440 375 5,5 1480 370 6 1500 370 6 The results show clearly the reaching of a stable point. The experiment was continued for several hours without the occurrence of an instability.

While in the description above,presently preferred embodiments illustrative of the present'invention have been described, it is to be understood that other embodiments of this invention may be made without departing from the true spirit and scope thereof.

What is claimed is:

1. In an electrically heated furnace, heating means comprising:

a first ceramic heat element; and

a second ceramic heat element connected in series across a spark gap with said first ceramic heat element.

2. The invention as set forth in claim 1 wherein said spark gap is formed by superimposed parts of said first and second heat elements.

3. The invention as set forth in claim 2 wherein said superimposed parts of said first and second heat elements have roughened surfaces.

4. The invention as set forth in claim ll wherein said first and second heat elements are elongated bars.

5. The invention as set forth in claim 4 wherein said bars are substantially rectangular in cross-sectional shape.

6. The'invention as set forth in claim 4 wherein said heat elements are curved to form semi-rings, said semirings being connected to form a coil-shaped furnace with one semi-ring resting upon and partially overlapping another semi-ring wherein the concave portions of said rings face one another and including installing range spacers connected between individual semirings.

7. The invention as set forth in claim 4 wherein said heat elements form semi rings and are connected in series with the front face of one ring contacting the front face of the adjacentring to form a coil.

8. In an electrically heated furnace, heating means comprising:

a plurality of bar-shaped ceramic heating elements, each of said heating elements being connected to another of said heating elements in series across a spark gap, said heating elements being stacked one on top of another, and including conductive connecting members of ceramic material located between adjacent pairs of said heating elements to space apart said heating elements.

9. The invention as set forth in claim 8 wherein each of said connecting members is integral with an adjoining heat element to form a substantially L-shaped element.

10. The invention as set forth in claim 8 including insulating range spacers adapted to space apart said L- shaped heating elements.

11. In an electrically heated furnace, heating means comprising:

a plurality of bar-shaped ceramic heating elements bent in the shape of semi-rings, said heating elements being stacked on top of one another in coilshaped fashion, partially overlapping one another, and including range spacers adapted to space adjacent pairs of said semi-rings apart.

12. The invention as set forth in claim 11 wherein said range spacers are substantially U-shaped, each of said spacers being comprised of two legs and a connecting cross-part to form said U-shape, wherein said range spacers are connected between said heating elements with the cross-part of each of said spacers extending away from the furnace. 

1. In an electrically heated furnace, heating means comprising: a first ceramic heat element; and a second ceramic heat element connected in series across a spark gap with said first ceramic heat element.
 2. The invention as set forth in claim 1 wherein said spark gap is formed by superimposed parts of said first and second heat elements.
 3. The invention as set forth in claim 2 wherein said superimposed parts of said first and second heat elements have roughened surfaces.
 4. The invention as set forth in claim 1 wherein said first and second heat elements are elongated bars.
 5. The invention as set forth in claim 4 wherein said bars are substantially rectangular in cross-sectional shape.
 6. The invention as set forth in claim 4 wherein said heat elements are curved to form semi-rings, said semi-rings being connected to form a coil-shaped furnace with one semi-ring resting upon and partially overlapping another semi-ring wherein the concave portions of said rings face one another and including installing range spacers connected between individual semi-rings.
 7. The invention as set forth in claim 4 wherein said heat elements form semi-rings and are connected in series with the front face of one ring contacting the front face of the adjacent ring to form a coil.
 8. In an electrically heated furnace, heating means comprising: a plurality of bar-shaped ceramic heating elements, each of said heating elements being connected to another of said heating elements in series across a spark gap, said heating elements being stacked one on top of another, and including conductive connecting members of ceramic material located between adjacent pairs of said heating elements to space apart said heating elements.
 9. The invention as set forth in claim 8 wherein each of said connecting members is integral with an adjoining heat element to form a substantially L-shaped element.
 10. The invention as set forth in claim 8 including insulating range spacers adapted to space apart said L-shaped heating elements.
 11. In an electrically heated furnace, heating means comprising: a plurality of bar-shaped ceramic heating elements bent in the shape of semi-rings, said heating elements being stacked on top of one another in coil-shaped fashion, partially overlapping one another, and including range spacers adapted to space adjacent pairs of said semi-rings apart.
 12. The invention as set forth in claim 11 wherein said range spacers are substantially U-shaped, each of said spacers being comprised of two legs and a connecting cross-part to form said U-shape, wherein said range spacers are connected between said heating elements with the cross-part of each of said spacers extending away from the furnace. 